Renewable energy is any type of energy which has an abundant and ongoing source, such as the Sun's rays, wind, waves, rivers, tides and the heat from radioactive decay in the Earth's core as well as biomass. Renewable energy does not include energy sources which are dependent upon limited and polluting resources, such as fossil fuels and nuclear fission power (collectively known as nuclear-fossil fuels).

Renewable energy may be used directly (as in solar ovens,
geothermal heat pumps, and windmills) or be used to
generate electricity or create fuels such
as ethanol. Throughout human existence, wood has been critically important as a thermal energy source. Historically, only the power of falling water in rivers (hydroelectricity) has been significantly tapped for the generation of electricity. However, recent years have seen the rapid development of wind generation farms by mainstream power companies. Solar energy's
main human application has been in agriculture and forestry, via
photosynthesis, and increasingly it is harnessed for heat.
Geothermal power can be used to generate electricity near hot spots in the Earth's crust. Biomass (e.g., sugar cane residue or biodiesel) is burned where it can be utilized.

Most renewable energy sources can trace their roots to solar energy, perhaps with the exception of geothermal[?] and tidal wave power[?]. For example, wind is caused by the sun heating the earth unevenly. Hot air is less dense, so it rises, causing cooler air to move in to replace it. Hydroelectric power can be ultimately traced to the sun too. When the sun evaporates water in the ocean, the vapor forms clouds which later fall on mountains as rain which is routed through turbines to generate electrity. The transformation goes from solar energy to potential energy to kinetic energy to electric energy.

Turning to the use of renewable energy sources for electricity, there
are immediate challenges in actually harnessing them. Apart from
photovoltaic (PV) systems, which turn sunlight directly into
electricity, the question is how to make them turn dynamos to
generate the electricity. If heat is used, a steam generating
system powers a turbine.

If the fundamental opportunity of renewables is their abundance and
relatively widespread occurrence, the fundamental problem, especially
for electricity supply, is their variable and diffuse nature (the
exception being geothermal energy, which is however only accessible
where the earth's crust is thin, such as near hot springs and natural geysers). This means either that there must be reliable overlapping sources of electricity, or some means of electricity storage on a large scale. Apart from pumped-storage hydro systems, no such means
exist at present, though use of hydrogen fuel cells is a distinct
possibility. For a stand-alone system the energy storage problem
remains paramount. If linking to a grid, the question of duplicate
sources arises. For large-scale and especially base-load electricity
generation there is little scope for harnessing the sun. However, for local producers, such as homeowners, using the electrical grid itself for storage is practical.

Renewable energy sources have a completely different set of
environmental costs and benefits than fossil fuel or nuclear power
plants. On the positive side they emit no carbon dioxide or other
air pollutants (beyond some decay products from new hydroelectric
reservoirs), but because they are harnessing relatively low-intensity
energy, their 'footprint' - the area they occupy - is
necessarily much larger. Whether large areas near cities dedicated to
solar collectors will be acceptable, if such proposals are ever made,
remains to be seen. Beyond utilising roofs, 1000 MWe of solar
capacity would require at least 20 square kilometres of collectors,
shading a lot of country. In Europe, wind turbines have not endeared
themselves to neighbours on aesthetic, noise or nature conservation
grounds, and this has slowed their deployment. However, European
non-fossil fuel obligations have led to increases in offshore wind
development. However, much of the environmental impact can be
reduced. Fixed solar collectors can double as noise barriers along
highways, roof-tops are available already, and there are places where
(redesigned) wind turbines would not obtrude unduly.

"Solar not nuclear" is a catch-cry of both antinuclear environmental
groups and many technological optimists, particularly as advances in
direct solar heating continue to be made. Certainly we can expect to
see more roof area occupied by some kind of solar collectors in the
future (especially for water heating), as their price comes down and
we adapt our energy usage to utilise better what is available from
this source. There are actually two separate approaches to solar energy, termed active solar and passive solar. The elements outlined below are included in active solar.

However, for electricity generation ground based solar power has limited
potential, as it is too diffuse and too intermittent. First, ground based solar input is interrupted by night and by cloud cover, which means that solar electric generation inevitably has a low capacity factor, typically less than 20%. Also, there is a low intensity of incoming radiation, and converting this to high grade electricity is still relatively inefficient (14% - 18%), though increased efficiency or lower production costs have been the subject of much research over several decades.

Two methods of converting the Sun's radiant energy to electricity are the focus of attention. The better known method uses sunlight acting on photovoltaic (PV) cells to produce electricity. This has application on satellites and for certain earthbound signalling and communication equipment, such as remote area telecommunications equipment. Sales of solar PV modules are increasing strongly as their efficiency increases and price diminishes. But the high cost per unit of electricity still rules out ordinary use.

Several experimental PV power plants mostly of 300 - 500 kW capacity are connected to electricity grids in Europe and the USA. Japan has 150 MWe installed. A large solar PV plant was planned for Crete. In 2001 the world total for PV electricity was less than 1000 MWe with Japan as the world's leading producer. Research continues into ways to make the actual solar collecting cells less expensive and more efficient. Other major research is investigating economic ways to store the energy which is collected from the Sun's rays during the day.

A typical solar thermal power plant has a system of mirrors to concentrate the sunlight on to an absorber, the resulting heat then being used to drive turbines. The concentrator is usually a long parabolic mirror trough oriented north-south, which tilts, tracking the Sun's path through the day. A black absorber tube is located at the focal point and converts the solar radiation to heat (about 400°C) which is transferred into a fluid such as synthetic oil. The oil can be used to heat buildings or water, or it can be used to drive a conventional turbine and generator. Several such installations in modules of 80 MW are now operating. Each module requires about 50 hectares of land and needs very precise engineering and control. These plants are supplemented by a gas-fired boiler which ensures full-time energy output. The gas generates about a quarter of the overall power output and keeps the system warm overnight. Over 800 MWe capacity worldwide has supplied about 80% of the total solar electricity to the
mid-1990s.

Alternatively, many individuals have installed small-scale PV arrays for domestic consumption. Some, particularly in isolated areas, are totally disconnected from the main power grid, and rely on a surplus of generation capacity combined with batteries and/or a fossil fuel generator to cover periods when the cells are not operating. Others in more settled areas remain connected to the grid, using the grid to obtain electricity when solar cells are not providing power, and selling their surplus back to the grid. This works reasonably well in many climates, as the peak time for energy consumption is on hot, sunny days where air conditioners are running and solar cells produce their maximum power output. Many states have passed "net metering" laws, requiring electrical utilities to buy the locally-generated electricity for price comparable to that sold to the household. Photovoltaic generation is still considerably more expensive for the consumer than grid electricity unless the usage site is sufficiently isolated, in which case photovoltaics become the less expensive.

A simple proposal for a solar electrical plant is the solar tower, in which a large area of land would be covered by a greenhouse made of something as simple as transparent foil, with a tall lightweight tower in the centre, which could also be composed largely of foil. The heated air would rush to and up the centre tower, spinning a turbine. A system of water pipes placed throughout the greenhouse would allow the capture of excess thermal energy, to be released throughout the night and thus providing 24-hour power production. A 200 MWe tower is proposed near Mildura, Australia[?].

The main role of solar energy in the future may be that of direct
heating. Much of our energy need is for heat below 60°C
(140°F) - e.g. in hot water systems. A lot more, particularly in
industry, is for heat in the range 60 - 110°C. Together these
may account for a significant proportion of primary energy use in
industrialised nations. The first need can readily be supplied by
solar power much of the time in some places, and the second
application commercially is probably not far off. Such uses will
diminish to some extent both the demand for electricity and the
consumption of fossil fuels, particularly if coupled with [[energy
conservation]] measures such as insulation.

Domestic solar hot water systems are common in the hotter areas of
Australia, and simply consist of a network of dark-coloured pipes
running beneath a window of heat-trapping glass. They typically have
a backup electric or gas heating unit for cloudy days. Such systems
can actually be justified purely on economic grounds, particularly in
some remoter areas of Australia where electricity is expensive.

With adequate insulation, heat pumps utilising the conventional
refrigeration cycle can be used to warm and cool buildings, with very
little energy input other than energy needed to run a compressor.
Eventually, up to ten percent of the total primary energy need in
industrialised countries may be supplied by direct solar thermal
techniques, and to some extent this will substitute for base-load
electrical energy.

Large scale solar thermal powerplants, as mentioned before, can be
used to heat buildings, but on a smaller scale solar ovens can be
used on sunny days. Such an oven or solar furnace uses mirrors or a
large lens to foccuss the Sun's rays onto a baking tray or black
pot which heats up as it would in a standard oven[?].

Wind turbines have been used for household electricity generation in
conjunction with battery storage over many decades in remote areas.
Generator units of more than 1 MWe are now functioning in several
countries. The power output is a function of the cube of the wind
speed, so such turbines require a wind in the range 3 to 25
metres/second (11 - 90 km/hr), and in practice relatively few
land areas have significant prevailing winds. Like solar, wind power
requires alternative power sources to cope with calmer periods.

There are now many thousands of wind turbines operating in
various parts of the world, with a total capacity of over 31,000 MWe of which Europe accounts for 75% (ultimo 2002). Germany is the leading producer of wind generated electricity with over 8000 MWe in 2001. In 2002 the U.S.A. produced over 4,200 Mwe of wind energy, second only to Germany.
New (offshore) wind parks are being planned and built all over the
world. This has been the most rapidly-growing means of electricity
generation at the turn of the 21st century and provides a valuable
complement to large-scale base-load power stations. Denmark generates
over 10% of its electricity with windturbines, whereas windturbines account for 0.4% of the total electricity production on a global scale (ultimo 2002). The most
economical and practical size of commercial wind turbines seems to be
around 600 kWe to 1 MWe, grouped into wind farms up to 6 MWe. Most
turbines operate at about 25% load factor over the course of a year,
but some reach 35%.

Where hot underground steam or water can be tapped and brought to the surface
it may be used to generate electricity. Such geothermal power sources have
potential in certain parts of the world such as New Zealand,
USA, Philippines and Italy. Some 8000 MWe of capacity is
operating. There are also prospects in certain other areas for
pumping water underground to very hot regions of the Earth's crust
and using the steam thus produced for electricity generation. An Australian startup company, Geodynamics, proposes to build a commercial plant in the Cooper Basin region of South Australia using this technology by 2004.

Hydroelectric power from potential energy of rivers, now supplies about 715,000 MWe or
19% of world electricity. Apart from a few countries with an
abundance of it, hydro capacity is normally applied to peak-load
demand, because it is so readily stopped and started. It is not a
major option for the future in the developed countries because most
major sites in these countries having potential for harnessing
gravity in this way are either being exploited already or are
unavailable for other reasons such as environmental considerations.

The chief advantage of hydro systems is their capacity to handle
seasonal (as well as daily) high peak loads. In practice the
utilisation of stored water is sometimes complicated by demands for
irrigation which may occur out of phase with peak electrical demands.

Harnessing the tides in a bay or estuary has been achieved in
France (since 1966) and Russia, and could be achieved in
certain other areas where there is a large tidal range. The trapped
water can be used to turn turbines as it is released through the
tidal barrage in either direction. Worldwide this technology appears
to have little potential, largely due to environmental constraints.

Harnessing power from wave motion is a possibility which might yield
much more energy than tides. The feasibility of this has been
investigated, particularly in the UK. Generators either coupled to
floating devices or turned by air displaced by waves in a hollow
concrete structure would produce electricity for delivery to shore.
Numerous practical problems have frustrated progress.

Ocean Thermal Energy Conversion is a relatively unproven technology, though it was first used by the French engineer Jacques Arsene d'Arsonval[?] in 1881. The difference in temperature between water near the surface and deeper water can be as much as 20°C. The warm water is used to make a liquid such as ammonia evaporate, causing it to expand. The expanding gas forces its way through turbines, after which it is condensed using the colder water and the cycle can begin again.

Biomatter or biomass can be used to produce biofuel ( bioalcohols -like methanol and ethanol- and biodiesel). Biodiesel can be used in nowadays petrol cars and itīs obtained for waste and crude vegetable and animal oil and fats (lipids).

All sorts of biomatter can be burnt to heat water and to drive
turbines. Plants partly use photosynthesis to store solar energy,
water and CO2. Sugar cane residue and other plant
matter can be, and is, burnt quite successfully. The process releases
no net CO2. Of course electricity is not the only form of practical energy. In some areas sugarbeets, cane and grasses are grown specifically to
produce ethanol (also known as alcohol) a liquid which can be used in internal combustion engines and fuel cells.

Sun, wind, tides and waves cannot be controlled to provide directly either continuous base-load power, or peak-load power when it is needed. In practical terms, without proper energy storage methods they are therefore limited to some 20% of the capacity of an electricity grid, and cannot directly be applied as economic substitutes for fossil fuels or nuclear power, however important they may become in particular areas with favourable conditions. Nevertheless, such technologies will to some extent contribute to the world's energy future, even if they are unsuitable for carrying the main burden of supply at this time.
If there were some way that large amounts of electricity from intermittent producers such as solar and wind could be stored efficiently, the contribution of these technologies to supplying base-load energy demand would be much greater. Already in some places pumped storage is used to even out the daily generating load by pumping water to a high storage dam during off-peak hours and weekends, using the excess base-load capacity from coal or nuclear sources. During peak hours this water can be used for hydroelectric generation.

Relatively few places have scope for pumped storage dams close to where the power is needed, and overall efficiency is low. Means of storing large amounts of electricity as such in giant batteries or by other means have not yet been developed. See hydrogen fuel cells below for a way to store energy. There is some scope for reversing the whole way we look at power supply, in its 24-hour, 7-day cycle, using peak load equipment simply to meet the daily peaks. Today's peak-load equipment could be used to some extent to provide infill capacity in a system relying heavily on renewables. The peak capacity would complement large-scale solar thermal and wind generation, providing power when they were unable to. Improved ability to predict the intermittent availability of wind enables better use of this resource. In Germany it is now possible to predict wind generation output with 90% certainty 24 hours ahead. This means that it is possible to deploy other plant more effectively so that the economic value of that wind contribution is greatly increased.

However, any substantial use of solar or wind for electricity in a grid means that there must be allowance for 100% back-up with hydro or fossil fuel capacity. This gives rise to very high generating costs by present standards, but in some places it may be the future.

Hydrogen is widely seen as a possible fuel for hydrogen cars, if certain problems can be overcome economically. It may be used in conventional internal combustion engines, or in fuel cells which convert chemical energy directly to electricity without flames, in the same way the human body burns fuel. Making hydrogen requires either reforming natural gas (methane) with steam, or, for a renewable and more ecologic source, the electrolysis of water into hydrogen and oxygen. The former process has carbon dioxide as a by-product, which exacerbates (or at least does not improve) greenhouse gas emissions relative to present technology. With electrolysis, the greenhouse burden depends on the source of the power, and both intermittent renewables and nuclear energy are considered here.

With intermittent renewables such as solar and wind, matching the output to grid demand is very difficult, and beyond about 20% of the total supply, apparently impossible. But if these sources are used for electricity to make hydrogen, then they can be utilised fully whenever they are available, opportunistically. Broadly speaking it does not matter when they cut in or out, the hydrogen is simply stored and used as required.

A quite different rationale applies to using nuclear energy for hydrogen. Here the plant would be run continuously at full capacity, with perhaps all the output being supplied to the grid in peak periods and any not needed to meet civil demand being used to make hydrogen at other times. This would mean maximum efficiency for the nuclear power plants and that hydrogen was made opportunistically when it suited the grid manager.

About 50 KWh (1/144,000 J) is required to produce a kilogram of hydrogen by electrolysis, so the cost of the electricity clearly is crucial.

Iceland is a world leader in renewable energy due to its abundant hydro and geothermal energy sources. Over 99% of the country's electricity is from renewable sources and most of its urban household heating is geothermal. Israel is also notable as much of its household hot water is heated by solar means. These countries' successes are at least partly based on their geographical advantages. The United States is the leading producer of hydroelectric power and geothermal electrical energy.

Share of the total power consumption in EU-countries that are renewable.